A Nest smoke and carbon monoxide detector is seen on Jan. 16, 2014, in Provo, Utah.

Breathing in and out is something our bodies do without our having to think about it. In fact, we rarely even give a second thought to the air surrounding us until the quality is low somehow – maybe from the smoke of a nearby fire or smog enveloping our crowded cities.

But there can be dangerous gases that are not always visible. Carbon monoxide and radon are two examples of deadly air contaminants that are completely invisible. We don't realize we're being poisoned from the air we breathe until it is too late. Luckily, these gases aren't that prevalent, and with the help of a little chemical sensing technology, we can let carbon monoxide and radon detectors do the worrying for us.

Chemical sensors are helpful far beyond just detecting deadly gases. These devices can be found in our homes, hospitals and in the military. There are many different types of sensors that detect different target molecules (also known as analytes). Although the sensors work in various ways, the gist is that a chemical interaction happens between the analyte and something in the sensor, and the device produces a measurable signal – a beeping or a color change to alert us to the presence of the target molecule.

Despite the differences in the construction of sensors, there are a few guiding principles that make any sensor good. The ideal one is inexpensive, foolproof and portable. Most importantly, any chemical sensor has two vital features: selectivity and sensitivity. There are more than 10 billion molecular substances in the world, so selectively detecting a single substance is no small feat [source: National Research Council]. Sensitivity is also incredibly important for detecting chemicals from a considerable distance or for trying to find very low concentrations of a target molecule. Other important sensor features are response time, packaging size and limit of detection — the lowest quantity of a substance that can be detected.

How Chemical Sensors Help
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No matter how they operate, chemical sensing devices are, without a doubt, working for you. Your home probably has at least a one detector for radon gas, smoke or carbon monoxide, depending on the laws in your state or country. Many radon sensors work by absorbing the radon itself or detecting the radioactive decay products of the lethal gas. Carbon monoxide, on the other hand, is not a radioactive material, so the detectors for this gas operate differently. One of the most common mechanisms for this chemical sensor is a riff on biology. These detectors mimic how carbon monoxide interacts with hemoglobin in blood in order to determine the presence of the gas. Another common detector in the home is a smoke detector. While some use radioactive materials to help sniff out smoke, most of the sensing in smoke detectors comes from the physical, not chemical, phenomenon of the smoke particles causing interference that is sensed by the detector.

Chemical sensing devices also have widespread use outside the home. One of the main places you'll see these devices in action is in search of biomolecules in medical settings. Biomolecule sensors are essentially specialized chemical sensors. Although they detect substances like hormones, these bodily substances are all molecules. After all, these sensors are made with many of the same guiding principles as other chemical sensors – selectivity, sensitivity and portability.

Some of the most portable biomolecule sensors you may be aware of are associated with fertility measurements: pregnancy tests and ovulation tests. Both these chemical sensors detect the presence of certain hormones in urine. In the case of pregnancy tests, the sensor looks for the hormone human chorionic gonadotropin (hCG) in urine. The stick on which the woman urinates has antibodies that are coated with a chemical that bonds to hCG. If the biomolecule is present, the test reads positive [source: Parents Magazine]. Usually these chemical sensors have a colorimetric component so that when the analyte – in this case hCG – binds, it triggers a color change in the sensor, making the readout of the results pretty foolproof.

In the clinical setting, two of the most common methods for chemical-based biomolecule detection are ELISA (enzyme-linked immunoabsorbent assay) and the Western blot. Depending on the size and type of the biomolecule in question and the information they want about the molecule, scientists and clinicians will often turn to one of these chemical sensing techniques to identify different analytes in mixtures of biomolecules [sources: ThermoFisher Scientific, Mahmood and Yang].

Sci-fi Detection Becoming a Reality

A glass head holds a neural interface sensor developed by DARPA, which reads signals from the brain and stimulates neurons to combat memory loss. DARPA is investing in chemical sensing research. Stacey Rupolo/Chicago Tribune/TNS via Getty Images

A glass head holds a neural interface sensor developed by DARPA, which reads signals from the brain and stimulates neurons to combat memory loss. DARPA is investing in chemical sensing research. STACEY RUPOLO/CHICAGO TRIBUNE/TNS VIA GETTY IMAGESA big driver for the future directions of chemical sensing is the military. Funding agencies like the U.S. Defense Advanced Research Projects Agency (DARPA) have big dreams for what chemical sensing might be able to do for its armed forces. Imagine being able to put a sensor on the top of a tent in a war zone that can detect explosives or toxic gases 4 miles (6 kilometers) away.

Or picture a sensor that is so tiny, it can be embedded into fabrics. When the sensor detects a toxic agent, the color of the fabric changes, alerting soldiers to the presence of chemical toxins in the air. What about a sensor that could to alert soldiers to their dehydration levels? The implications for such technology would not only be the upper hand in battle, but also save potentially thousands of lives.

Author's Note: How Chemical Sensing Devices Work
Nanoparticle sensors sewn into fabric that could tell me more about myself or the air around me? This is seriously the stuff of sci-fi. Except it's not. Maybe the military is the one pushing the research forward on things like this, but soon after soldiers start using it, it'll be commonplace for the rest of us. Gives wearable technology a whole new meaning.top terrorism by detecting minute concentrations of explosives without us having to undergo the extensive hassles of the security lines. In fact, scientists have already unveiled a sensor that can rapidly detect drugs and explosives close to 100 feet (30 meters) away with airports in mind [source: Engineering360]. These sensors could be installed at airport entrances and other places with large groups of people. Some types of chemical sensing devices can even be attached to cell phones to scan larger areas as law enforcement officials move around [source: Hsu].

Of course, chemical sensors don't only need to be used to divert nefarious activities. Detecting metals contaminants in water and possibly toxic chemicals in the air via sensors in security badges are just some of the ways that chemical sensing devices are changing our world.